1. Field of the Invention
The invention relates generally to memory systems and more particularly to solid state memories which require periodic refreshing of the information stored therein.
2. Description of the Prior Art
One of the major problems in a semiconductor industry is increasing the yield of semiconductor device fabrication processes. Unlike many industries where substantially 100 percent yield can be guaranteed, the semiconductor industry constantly faces yield problems. In many processes 25 percent yield is considered excellent yield. Conseuquently, any improvement in process yields has a startling effect on production capacity of a semiconductor plant without requiring any further capital investment.
Semiconductor manufacturing yields are generally poor on a percentage basis because of the samll size and fine detail of the devices being produced. Slight variations in process conditions, some of them virtually undetectable, frequently produce measurable changes in the device characteristics and in many cases render the final device inoperative. While yield can be accounted for in the final cost of individual devices, poor device yield within a large scale integrated (LSI) circuit is compounded because a single faulty device renders the entire circuit useless even though it contains many good devices. Consequently, the semiconductor industry continually strives to increase the yield of individual devices within an integrated circuit. Alternatively, the yield of the integrated circuits may be increased by changes in circuit operation.
Each device must be kept as small as possible in large scale integrated circuits in order to accommodate a large number of devices in the area of a die. This is particularly true where a single device, for example memory cells in LSI circuits, are duplicated many times on the die. It is common, for decoding simplicity, to make the number of memory storage cells on a die a power of two. Memories are now being produced with 1,024 memory cells per die, with chips measuring 140 mils by 140 mils. In order to achieve such memory cell densities, it has been necessary to shrink the cell as much as possible. With this objective, memories which require periodic refreshing of the information stored therein have been developed because they result in memory cells which utilize less semiconductor area than memory cells which do not require refreshing. Such memory cells which are self-refreshing from clock signals are considered preferable to those cells which must be periodically re-written because the self-refreshing cells provide a more versatile memory.